The benefits of four chamber pacing have become widely apparent in recent years, and have generally been disclosed and published in the literature. Cazeau et al., PACE, vol.17, November 1994, Part II, pp. 1974-1979, disclose investigations leading to the conclusion that four-chamber pacing is feasible; and that in patients with evidence of interventricular dyssynchrony a better mechanical activation process can be obtained by resynchronizing the polarization of the right and left ventricles, and optimizing the AV sequence on both sides of the heart. In the patent literature, U.S. Pat. No. 4,928,688 is representative of a system for simultaneous left ventricular (LV) and right ventricular (RV) pacing, known as "bi-ventricular" pacing. In this system, natural ventricular depolarizations are sensed in both chambers, and if one chamber contracts but the other one does not within a short time interval, then the non-contracting chamber is paced.
Bi-ventricular pacing, or more generally four-chamber pacing, is a response to the ongoing quest to provide better pacing therapy for patients with congestive heart failure (CHF). CHF is defined generally as the inability of the heart to deliver enough blood to the peripheral tissues to meet metabolic demands. Frequently CHF is manifested by left heart dysfunction, but it can have a variety of sources. For example, CHF patients may have any one of several different conduction defects. The natural electrical activation system through the heart involves sequential events starting with the sino-atrial (SA) node, and continuing through the atrial conduction pathways of Bachmann's bundle and internodal tracts at the atrial level, followed by the atrio-ventricular (AV) node, Common Bundle of His, right and left bundle branches, and final distribution to the distal myocardial terminals via the Purkinje fiber network. A common type of intra-atrial conduction defect is known as intra-atrial block (IAB), a condition where the atrial activation is delayed in getting from the right atrium to the left atrium. In left bundle branch block (LBBB) and right bundle branch block (RBBB), the activation signals are not conducted in a normal fashion along the right or left bundle branches respectively. Thus, in a patient with bundle branch block, the activation of the ventricle is slowed, and the QRS is seen to widen due to the increased time for the activation to traverse the conduction path. For example, in a patient with LBBB, the delay in the excitation from the RV to the LV can be as high as 120 to 150 ms.
As used herein, the term bi-ventricular pacing refers to simultaneous or substantially simultaneous pacing of the two ventricles. Thus, pulses of exactly the same timing may be delivered to each ventricle, or, e.g., the pulse to the left ventricle may be delivered one or two ms before the pulse to the right ventricle. Likewise, it is known that there is an advantage to simultaneous or nearly simultaneous pacing of the left atrium and the right atrium for patients with IAB. In a patient with IAB, the left atrium may be excited to contract up to 90 ms later than the right atrium. It can be seen that if contractions in the left ventricle and the right ventricle are triggered at about the same time, then the left AV synchrony is impaired, with the left ventricle not having adequate time to fill up. The advantage of pacing the two atria for patients with IAB is disclosed at AHA, 1991, Abstract from 64th Scientific Sessions, "Simultaneous Dual Atrium Pacing in High Degree Inter-Atrial Blocks: Hemodynamic Results," Daubert et al., No. 1804. Thus, a pacing system may provide dual site pacing of the two ventricles, or the two atria, or both. Consequently, as used herein, the term "dual site" refers to pacing substantially simultaneously, or concurrently in either the ventricles or the atria.
It is recognized, of course, that dual site pacing will, as a general proposition, require roughly twice as much pulse power delivery to the patient's heart as corresponding single chamber pacing. In view of this, it becomes doubly important to ensure that the pacemaker reliably causes the myocardium of the heart chamber to contract or "beat," i.e., to "capture" the heart. Stimulation pulses, or pacing pulses provided by an implanted pacemaker, have well defined amplitude and pulse width characteristics which can be adjusted by remote programming and telemetry equipment, or automatically by the pacemaker, to meet physiologic and device power conservation needs of the particular patient in which the pacemaker is implanted. As used herein, the term "pulse output," or PO, refers to the energy of the delivered pacing pulse, which can be varied by adjusting the amplitude or the pulse width, or both.
The amplitude (strength) and pulse width (duration) of the pacing pulses must be of such an energy magnitude above the stimulation threshold that capture is maintained, in order to prevent serious complications and even death. Moreover, it is desirable that this energy is not higher than a reasonable "safety margin" above the stimulation threshold, in order to prolong battery life. The patient's stimulation thresholds in the different chambers often fluctuate in the short term, and gradually change in the long term. For dual site pacing, where the positioning of the pacing electrodes is quite different, e.g., comparing the left ventricle to the right ventricle, these thresholds will not be exactly the same. While the safety margin for each heart chamber to be paced is typically set by the physician at the time of implantation of the pacemaker system to account for projected chronic thresholds, there remains a need for chronically testing threshold, so as to track any changes in threshold and continually adjust PO to a proper level.
In prior art single chamber and dual chamber pacing systems, a great deal of effort has been expended to develop pacemakers having the capability of automatically testing the stimulation threshold, i.e., providing an "auto-capture" detection function, and resetting the pacing pulse energy to exceed the threshold by the safety margin without the need for clinical or patient intervention. A wide variety of approaches have been taken in the pacemaker art as reflected by the patent literature. See, for example, U.S. Pat. Nos. 5,324,310; 5,320,643; 5,165,404; 5,165,405; 5,172,690; 5,222,493; and 5,285,780. The capture detection threshold tracking approaches in the prior art have taken a variety of forms, and typically attempt to overcome the difficulty in detecting the evoked cardiac response wave shape from the pacing electrodes employed to deliver the pacing pulse. High stimulation energy pacing pulses and the ensuing after potentials and electrode-tissue polarization artifacts mask the evoked response, and also saturate the sense amplifiers coupled to the electrodes, until they dissipate. By the time that the sense amplifier is no longer "blinded," the evoked response, if any, has typically passed the electrodes. As a consequence, most of the prior art approaches rely on additional components and circuitry, and more complex logic, which consume energy, add to the bulk and cost of the system, and also raise reliability issues. This situation is, of course, worsened in the case of dual site pacing systems. There is thus an aggravated need in the area of dual site pacemakers to provide a reliable technique for determining whenever there is loss of capture (LOC) in either chamber, without compounding the circuitry and logic complexity of the pacemaker.